.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
.\"
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.\" Permission is granted to distribute possibly modified copies
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.\" $Id: rtnetlink.3,v 1.2 1999/05/18 10:35:10 freitag Exp $
.\"
.TH rtnetlink 3 2022-10-30 "Linux man-pages 6.03"
.SH NAME
rtnetlink \- macros to manipulate rtnetlink messages
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <asm/types.h>
.B #include <linux/netlink.h>
.B #include <linux/rtnetlink.h>
.B #include <sys/socket.h>
.PP
.BI "rtnetlink_socket = socket(AF_NETLINK, int " socket_type \
", NETLINK_ROUTE);"
.PP
.BI "int RTA_OK(struct rtattr *" rta ", int " rtabuflen );
.PP
.BI "void *RTA_DATA(struct rtattr *" rta );
.BI "unsigned int RTA_PAYLOAD(struct rtattr *" rta );
.PP
.BI "struct rtattr *RTA_NEXT(struct rtattr *" rta \
", unsigned int " rtabuflen );
.PP
.BI "unsigned int RTA_LENGTH(unsigned int " length );
.BI "unsigned int RTA_SPACE(unsigned int "length );
.fi
.SH DESCRIPTION
All
.BR rtnetlink (7)
messages consist of a
.BR netlink (7)
message header and appended attributes.
The attributes should be manipulated only using the macros provided here.
.PP
.BI RTA_OK( rta ", " attrlen )
returns true if
.I rta
points to a valid routing attribute;
.I attrlen
is the running length of the attribute buffer.
When not true then you must assume there are no more attributes in the
message, even if
.I attrlen
is nonzero.
.PP
.BI RTA_DATA( rta )
returns a pointer to the start of this attribute's data.
.PP
.BI RTA_PAYLOAD( rta )
returns the length of this attribute's data.
.PP
.BI RTA_NEXT( rta ", " attrlen )
gets the next attribute after
.IR rta .
Calling this macro will update
.IR attrlen .
You should use
.B RTA_OK
to check the validity of the returned pointer.
.PP
.BI RTA_LENGTH( len )
returns the length which is required for
.I len
bytes of data plus the header.
.PP
.BI RTA_SPACE( len )
returns the amount of space which will be needed in a message with
.I len
bytes of data.
.SH STANDARDS
These macros are nonstandard Linux extensions.
.SH BUGS
This manual page is incomplete.
.SH EXAMPLES
.\" FIXME . ? would be better to use libnetlink in the EXAMPLE code here
Creating a rtnetlink message to set the MTU of a device:
.PP
.in +4n
.EX
#include <linux/rtnetlink.h>

\&...

struct {
    struct nlmsghdr  nh;
    struct ifinfomsg if;
    char             attrbuf[512];
} req;

struct rtattr *rta;
unsigned int mtu = 1000;

int rtnetlink_sk = socket(AF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE);

memset(&req, 0, sizeof(req));
req.nh.nlmsg_len = NLMSG_LENGTH(sizeof(req.if));
req.nh.nlmsg_flags = NLM_F_REQUEST;
req.nh.nlmsg_type = RTM_NEWLINK;
req.if.ifi_family = AF_UNSPEC;
req.if.ifi_index = INTERFACE_INDEX;
req.if.ifi_change = 0xffffffff; /* ??? */
rta = (struct rtattr *)(((char *) &req) +
                         NLMSG_ALIGN(req.nh.nlmsg_len));
rta\->rta_type = IFLA_MTU;
rta\->rta_len = RTA_LENGTH(sizeof(mtu));
req.nh.nlmsg_len = NLMSG_ALIGN(req.nh.nlmsg_len) +
                              RTA_LENGTH(sizeof(mtu));
memcpy(RTA_DATA(rta), &mtu, sizeof(mtu));
send(rtnetlink_sk, &req, req.nh.nlmsg_len, 0);
.EE
.in
.SH SEE ALSO
.BR netlink (3),
.BR netlink (7),
.BR rtnetlink (7)
